1. Field of the Invention
The present invention relates to an image transmission apparatus used for television conference systems, television telephone systems, and the like.
2. Description of the Prior Art
FIG. 1 shows in block diagram the image transmission apparatus based on the DCT scheme which is derived from the block diagram of the image processing apparatus based on the VQ scheme disclosed in publication "Mitsubishi Electric Corp. Technical Bulletin", Vol. 60, No. 10, p. 40, published in 1986. In the figure, indicated by 1 is an image pickup camera, 2 is an A/D converter which converts the analog video signal in NTSC format produced by the camera 1 into a digital video signal, 3 is a CIF converter which converts the digital video signal from the A/D converter 2 into a signal of CIF format, 4 is a sending frame memory which stores the CIF-formatted video signal provided by the CIF converter 3, 5a is a subtracter which evaluates the difference between the video signal read out of the sending frame memory 4 and the decoded video signal of the previous frame (a video signal from a movement compensating filter 12 which will be explained shortly), 5 is a DCT encoder which implements DCT-encoding for the differential signal from the subtracter 5a, 6 is a variable-length encoder which encodes the video signal from the DCT encoder 5, 7 is a sending buffer which temporarily holds the video signal from the variable-length encoder 6, 8 is a multiplexer which multiplexes the video signal from the sending buffer 7 with other data, and 9 is a line interface which transmits the multiplexed data from the multiplexer 8 over a line.
Indicated by 10 is a local decoder which decodes the video signal that has been DCT-encoded by the DCT encoder 5, 11a is an adder which merges the video signal from the local decoder 10 with the decoded video signal of the previous frame (a video signal from the movement compensating filter 12), 11 is an intra-loop frame memory which temporarily stores the merged video signal, and 12 is a movement compensating filter which implements compensating the movement and filtering for picture quality improvement based on the video signal from the intra-loop frame memory 11 and the video signal from the sending frame memory 4.
Indicated by 13 is a line interface for receiving data over the line, 14 is a separator which separates multiplexed data provided by the line interface 13, 15 is a variable-length decoder which decodes the video signal from the separator 14, 16 is a receiving buffer which temporarily holds the video signal from the variable-length decoder 15, 17 is a DCT decoder which decodes the video signal from the receiving buffer 16, 18 is a movement decoder which decodes the movement, 19a is an adder which the merges video signal from the DCT decoder 17 with the video signal from the movement decoder 18, and 19 is an intra-loop frame memory which temporarily stores the result of summation for the implementation of movement decoding
Indicated by 20 is a receiving frame memory which temporarily stores the same contents as of the intra-loop frame memory 19 for the implementation of CIF inverse conversion, 21 is a CIF inverse converter which inverts the video signal from the receiving frame memory 20, 22 is a D/A converter which converts the digital video signal from the CIF inverse converter 21 into an analog video signal, 23 is a monitor panel for displaying the video signal provided by the D/A converter 22, and 24 is a system controller which controls the overall apparatus.
Indicated by 112 is an A/D turn loop which is selected manually through the command of the system controller 24, 102 is a FIL turn loop which is selected manually through the command of the system controller 24, 103 is a LDC turn loop which is selected manually through the command of the system controller 24, 104 is a BM turn loop which is selected manually through the command of the system controller 24, and 105 is a OA turn loop path which is selected manually through the command of the system controller 24.
Next, the operation of the foregoing image transmission apparatus will be explained, first for the signal path in the normal operation and next for the loop paths.
During the normal operation, the video signal produced by the camera 1 is fed to the A/D converter 2, by which the analog video signal is converted into the digital video signal. The digital video signal of NTSC format is converted into the signal of CIF format, which is the world standard video format, by the CIF converter 3 and then the resulting data is stored temporarily in the sending frame memory 4.
The subtracter 5a evaluates the difference between the video signal read out of the sending frame memory 4 and the video signal of the previous frame which has been rendered coding and decoding, and the differential signal is DCT-coded by the DCT encoder 5. The DCT-coded video signal is coded by the variable-length encoder 6, and at the same time it is sent to the local decoder 10 for DCT decoding. The DCT-decoded video signal is merged by the adder 11a with the coded-and-decoded video signal of the previous frame which is provided by the movement compensating filter 12, and the resulting data is stored temporarily in the intra-loop frame memory 11. The video signal stored in the intra-loop frame memory 11 is compared with the video signal stored in the sending frame memory 4 by the movement compensating filter 12, by which the signal is rendered the movement compensation and then filtering for the enhancement of picture quality, and the video signal derived from that of the previous frame, with coding and decoding being done, is produced.
The video signal coded by the variable-length encoder 6 is fed through the sending buffer 7 and multiplexed with other data (not shown in FIG. 1) by the multiplexer 8. The multiplexed data is transmitted over the line through the line interface 9.
Multiplexed data coming over the line is received by the line interface 13, and separated into a video signal and other data by the separator 14. The separated video signal is decoded by the variable-length decoder 15, fed through the receiving buffer 16, and decoded by the DCT decoder 17. The DCT-decoded video signal is merged with the video signal of the previous frame by the adder 19a, and it is stored temporarily in the receiving frame memory 20 and also stored temporarily in the intra-loop frame memory 19. The video signal read out of the intra-loop frame memory 19 is rendered the movement decoding (compensation) by the movement decoder 18, and a video signal of the previous frame is produced.
The video signal held in the receiving frame memory 20 has its CIF format converted into NTSC format by the CIF inverse converter 21, and it is further converted into an analog video signal by the D/A converter 22 and delivered to the monitor panel 23. These operations are controlled by the system controller 24.
The loop paths are used to test the operations of the sections of the image transmission apparatus. The loop pathes include the A/D turn loop 112, FIL turn loop 102, LDC turn loop 103, BM turn loop 104 and OA turn loop 105, and any of the loop paths is activated manually through the system controller 24.
With the A/D turn loop 112 being active, the analog video signal provided by the camera 1 is once converted into a digital video signal by the A/D converter 2, and then immediately converted back to a digital video signal by the D/A converter 22 and delivered to the monitor panel 23. With the FIL turn loop 102 being active, the video signal at the output of the sending frame memory 4 is shunted to the receiving frame memory 20. With the LDC turn loop 103 being active, the video signal at the output of the DCT encoder 5 is fed through the local encoder 10, intra-loop frame memory 11 and movement compensating filter 12, and shunted to the receiving frame memory 20. With the BM turn loop 104 being active, the video signal at the output of the sending buffer memory 7 is shunted to the variable-length decoder 15. With the OA turn loop 105 being active, the video signal at the output of the line interface 9 is shunted to the line interface 13.
These five test loops are used in the event of failure of the image transmission apparatus for finding a possible faulty section based on the judgement procedure shown in FIG. 2. Referring to FIG. 2, if the A/D turn loop test through the OA turn loop test have all terminated normally, then the image transmission apparatus under test is normal. If the A/D turn loop test through the LDC turn loop test have terminated normally and the remaining loop tests have revealed abnormalities, then a section after the variable-length encoder 16 will be defective. If the A/D turn loop test and FIL turn loop test have terminated normally and the remaining turn loop tests have revealed abnormalities, then a section after the. DCT encoder 10 will be defective. If only the A/D turn loop test has terminated normally and the remaining turn loop tests have revealed abnormalities, then a section after the CIF converter 3 will be defective. If all loop tests have revealed abnormalities, then a section after the A/D converter 2 will be defective. These loop tests are conducted by activating the test loops sequentially from the A/D turn loop 112 up to the OA turn loop 105, while the staff of test watches the monitor panel 23 for the judgement of each test.
The conventional image transmission apparatus is arranged as described above, and it necessitates a staff of test for the diagnostic operation of the apparatus. The diagnostic scheme is limited in that the test staff selects each turn loop manually and observes the response on the monitor panel 23, and the apparatus lacks in the ability of automatic self-diagnosis.